LARYNGEAL SENSORY RECEPTORS

The larynx has a rich sensory innervation, as
suggested by the fact that in the cat, the internal
branch of the SLN contains approximately 2200
myelinated afferent fibers, whereas the cervical vagus,
which supplies the larger área of the thorax and
abdomen, has about 3000. These afferent fibers carry
information from both mechanoreceptors and
chemoreceptors in the larynx.
In the cat, mechanoreceptors have been divided into
more superficial mucosal “touch” receptors and “deep”
receptors thought to be in laryngeal muscles or
joints. Sant Ambrogio and colleagues recorded action
potentials from the SLN in dogs spontaneously
breathing through either the upper airway or a
tracheotomy and found evidence of several classes of
mechanoreceptors. “Pressure” receptors were thought
to respond to positive, or more frequently negative,
changes in transmural pressure, whereas “flow”
receptors were triggered by changes in airflow. In the
absence of changes in airflow and pressure, signáis
from “drive” receptors were noted that were thought to
be the result of contraction of the laryngeal muscles.
The “flow” receptors also responded to cooling of the
laryngeal mucosa. Proprioceptive input also appears
to play a role, as suggested by histologic studies of
human larynges, in which muscle spindles were found
in all of the intrinsic muscles of the larynx except the
LCA.
Actual chemoreceptors have yet to be identified in
the laryngeal mucosa, but experiments have
demonstrated laryngeal chemosensitivity in a number
of different species. Boushey et al identified two groups
of laryngeal receptors in the cat, both of which responded
to stimulation with ammonia or water,
whereas other substances such as C02, S02, and
cigarette smoke produced a response in only one type
of receptor.3 Interestingly, saline produced no
re-sponse. Other researchers have found evidence of laryngeal
receptors sensitive to milk, gastric contents,
saliva, and isotonic sugars as well as water in a
variety of animals. They found that isotonic solutions
other than saline were capable of provoking the laryngeal
chemoreflex and postulated that the lack of
chloride ions might trigger the laryngeal
chemoreceptor. Laryngeal chemosensitivity to water,
however, has provoked the most interest among
researchers, especially because of the reflex apnea
seen in immature animals when water is applied to the
laryngeal mucosa. This reflex may play a role in the
pathophysiology of SIDS. Work has shown that
lambs infected with respiratory syncytial virus (RSV)
seem to have an exaggerated respiratory depression
in response to laryngeal chemostimulation.
Interestingly the peak incidence of SIDS corresponds
to that of RSV, whereas RSV is also known to be
associated with apnea in young infants.